1. Bispecific antibodies with specificity for TcR gamma/delta chains can induce human PBL subsets to lyse surrogate target cells. Not all targetable activity is in the gamma/delta + T cells. 2. Targeted human PBL exhibited an anti-tumor activity mediated by factors secreted into the medium. This activity leads to the eradication of bystander tumor cells, and appears to operate both in vitro and in vivo. 3. Targeted human PBL can block intraperitoneal growth of established human ovarian carcinoma in nude mice. The potency of the effect is donor dependent, and varies with the type of bispecific antibody used, the mode of activation of the PBL, and the treatment schedule.